'71 Differential Ratio

[WhiteKnight]

I think I have figured out what ratio should be in the differential, but I am looking for help as I am uncertain from the stampings. I think it is a 3.36

1AX 12 6 68 W is stamped on the botttom.

-steve

 

[GTR1999]

AX is the ratio code and that looks like 1968? If you have it out divide out the teeth on the R&P otherwise count the # of turns for the driveshaft to wheel. 3=308 3 1/3= 336, 3 1/2= 355 3 3/4= 370
If it's a 68 housing then who knows what's in there. What does the tach read at 60 mph?
The base engines with M-20 had 336 gears.

Gary

 

[WhiteKnight]

That was my same thought too, it looks like a it may be from a 68. I will have to try counting the number of turns since the car also has other non-standard parts installed. The distributor is a HEI unit with electronic tach output. My mechanical tach is not connected. At least the cable is still there and I was able to spin it by hand and make the tach move, but who knows if it will still function correctly once I get a replacment distributor that has the mechanical drive.

Just one other thing to fix, like so many other small things on this car. I just finished "repairing" the oil pressure gauge, it really only needed a little cleaning. One of the shafts for the gear that moves the indicator needle was corroded and stuck. A little machinists oil and working it throught the range fixed that. The Q-Jet has the #7043201 (1973 454 manual trans) and the alternator looks to be a replacment for passenger cars from '75. 1102480 61-A 6E 5-12VNEG.

I think it may be possibly higher than a 3.36 since on my trip home from Atlanta, where I purchased it, I got 19.9 MPG @ 75-80.

Thanks Again, I'll let you know what I find out.

It's a base engine with a TH400 (both are matching#'s for casting/date code and VIN).


-steve

 

[WhiteKnight]

The drive shaft rotated 3 1/3 to one turn of the wheel. It was more than three but not a full half. The stamping is a little odd, I looked closer at it an it looks like the "68" may be a "G" stamped three times. Looks like the first one was inverted then they tried to correct it but it came out as an 8. Then they stamped it again to the left of the double G and now it looks like a 68. Here is a picture.


I did notice another problem the passenger side rear rim is rubbing ever so slightly on the bushing for the outside mounting bolt of the rear spring. The driver side has a 1/2 inch of clearance. Anyone ever seen this problem?

 

[pgtr]

I agree w Gary - the stamp indicates 3.36 but probably from a 1969 model. The W might indicate the posi brand. And if so it's not likely but arguably a few 69s still may have seen production w/o posi. Extremly unlikely however. Obviously you want posi. The 3.36 ratio is a good all around ratio IMHO for a vintage C3.

On your tach there is available an electronic conversion for the dash that, as I understand it, keeps your needle and face and hooks up with a wire. You've already got a newer electronic ignition that supercedes so that might be an option to consider...

 

[pgtr]

I agree w Gary - the stamp indicates 3.36 but probably from a 1969 model. The W might indicate the posi brand. And if so it's not likely but arguably a few 69s still may have seen production w/o posi. Extremly unlikely however. Obviously you want posi. The 3.36 ratio is a good all around ratio IMHO for a vintage C3.

On your tach there is available an electronic conversion for the dash that, as I understand it, keeps your needle and face and hooks up with a wire. You've already got a newer electronic ignition that supercedes so that might be an option to consider...

 

[Jack]

First, welcome aboard CAC!
19+ mpg seems high ... even for 71 base L48 on long road trip. If it's really got a 3.36 ... suspect speedo gear inside trans is wrong one for 3.36 ... that both speedo & odometer are reading high.

Rear clearance /rubbing? ... possible: camber adjustment out, strut rod bushing(s) shot etc.

If & when you decide to go for a distributor that'll run your OE mech tach ... and if you don't require numbers-match-correct ... lemme know. Just click on my membername Jack at left ... it gives option to email me thru CAC.

 

[JohnZ]

The "W" stands for the Chevrolet-Warren plant, where all Corvette diffs were built.

 

[WhiteKnight]

Thank for all the help. On the rubbing, I think it may have been from being jacked up to do the spin test. I backed down the drive and checked again, it had settled back to a "normal" comfortable clearance.

I could be a little off on the milage calculation but I did ussully stop every 160-195 miles and put 8-10 galons in the tank, and a quart of oil in the motor. I was not really pushing and keeping out of the throttle, not hard launches and gradually getting up to speed. I was a little concerned of breakage since I had bought it sight unseen (pics on the net) and it was going to be shipped but Rita came through and set the date back by two weeks. So I flew out and drove her home.

 

[Jack]

WK:
Someone please correct me if wrong ... but I believe base 71 L48 350 w/ TH400 auto trans came w/ 3.08 gear. Have friend w/ new vehicle hold 50 mph on freeway while you pull alongside and note your speedo ... that'll tell you a lot. If you have a functional tach ... what's it read at 50 mph? ... at 60 mph?

If trans' speedo drive gear is for 3.08 reargear but you have 3.36 ... your speedo and odometer will read high by about +9 to +10 percent ... when your odometer says you've been 175 mi, you've really gone about 160 ... when your speedo says you're running 75mph, you're really running about 69. If this were the case, any gas mileage calculated from face value would be suspect.
JACK:gap

 

[GTR1999]

As Jack said, usually the base TH400 cars came with 308's. It's not a bad setup for a cruiser. My 72 is a base w/TH400 and I kept the 308's when I rebuilt it. I wanted the highway gear and this is just for driving. I setup 373 gears in my 4 speed 69 and that's a nice compromise, although the 336's were fine for combined driving too.
Sounds like your car has been apart,not unusal for a 30 year old car still on the road. If you're going to do any work on the differential let me know and I'll help you out. Remember too if you change ratios you'll have to change the speedo gear to keep it in range.
I like the old points tach drive distributors.I've rebuilt a few for fellow vetters and run them in 2/3 of our vettes. All have been blueprinted to tighter spec's then the factory and run great. With the original tach housing I open up the oiling holes in the body so that I get better oiling on the bushings and can install it with the vacuum can pointing to #8 cyl for better cable alignment.
Good luck with your car and PM me if I can help.
Gary

 

[WhiteKnight]

Many of these same ideas have crossed my mind, mainly is the speed correct and the milage on the OD rolling accurate. Both from a stand point of not wanting to get a ticket due to a speedo error and since I have collector car ins with a milage limit I want that to be accurate too.

I have checked my speed against some of the radar display devices that the police setup on streets and around construction areas. 35 appears to be 35 and 45 on the speedo looks to be correct too. I have also held it at 60 over a measured (mile marker) mile and it took 60 seconds to travel.

I have a '03 Tahoe and my wife has a '01, we'll have to check it against those at various speeds.

I'm fairly certain it does have a posi from they way it hooks up both tires on a hard launch. Also when I did the spin check to calc the ratio the tire I was not turning spun in the oposite direction. Additionally with the strut rod bushing rubbing on that oposite (passenger side) rim, when the wheel balance weight made contact with the bushing I was not able to turn the drivers side wheel. It also has the red warning label for posi fluid on the diff.

I'm sure that my milage will drop considerably now that I am driving it as a vette should be driven and I don't plan to take any more 950 mile (one way) trips.

 

[lone73]

Hey WhiteKnight, I see you're in Austin. I'm just south of Austin - West of Buda a few miles. I didn't think there were any regular posters on this board from the area. What part of town are you in?

later
.
.
.

 

[WhiteKnight]

I live in Pflugerville just north of Austin. You can send me a PM or e-mail me just click on my name at left ... it gives option to email me thru CAC.

I just pickup my baby on Sept 30th so I'm new to the board and it is my first corvette.

 

[TWINRAY]

My '69 has the 3.08 rear and runs at 3000 rpm at 80 mph. I get at least 20 mpg cruising at speeds up to 75 mph but with speeds normally running probably at around 65. That's what it delivered on my 600 round trip run to Watkins Glen in September. With the TH400, it's a great combination if you like to cruise and I wouldn't call it a slug off the line either. It all depends what you're looking for.

 

[WhiteKnight]

Thanks Twinray. I'm not sure what I'm really looking to find out other than just to know more about the car. I've had it a little over 4 weeks now. Since there is so much information available that you can learn from the casting numbers, I was checking everything. What did confuse me and sparked the inital question was the AX code (optional ration on the base motor/auto) with a date that should be in a '69 model year. However, as GTR1999 said, its not unusual for a 30+ year old car to have been apart.

What I have learned is that it is a 3.36 ratio posi and if not that way from the factory then a prior owner changed it along with the speedo drive gear. Since the vette's speedometer did match speeds on my newer vehicles up to 80 mph checked in 10 mph increments. I also know that since the tranny and diff are working correctly it is better to focus on building a motor with a nice fat torque curve rather than a high rpm screamer.

-steve
"If it 'aint broke I can't fix it"

 

[pgtr]

... it is better to focus on building a motor with a nice fat torque curve rather than a high rpm screamer.

I have to stop dinkign with stuff that ain't broke too. But yeah - I too like that kind of approach to engine buildups (or is that build 'outs' ? ). I run a Crane cam that is probably basically an 'RV' cam as some might call it. One can probably do better nowadays with dual profiles or rollers etc. It offers lots of torque off idle on up into the mid 4K RPM range as I recall.